1. Field of the Invention
This invention relates to a radiation imaging panel adapted for use in a radiation imaging apparatus, such as an X-ray imaging apparatus. This invention particularly relates to a photo-conductor layer for constituting the radiation imaging panel.
2. Description of the Related Art There have heretofore been proposed X-ray imaging panels designed for use in a medical X-ray image recording operation, such that a radiation dose delivered to an object during the medical X-ray image recording operation may be kept small, and such that the image quality of an image and its capability of serving as an effective tool in, particularly the efficient and accurate diagnosis of an illness may be enhanced. With the proposed X-ray imaging panels, a photo-conductor layer sensitive to X-rays is employed as a photosensitive material. The photo-conductor layer is exposed to X-rays carrying X-ray image information, and an electrostatic latent image is thereby formed on the photo-conductor layer. Thereafter, the electrostatic latent image, which has been formed on the photo-conductor layer, is read out by use of light or a plurality of electrodes. The techniques utilizing the X-ray imaging panels have advantages over the known photo-fluorography utilizing TV image pickup tubes in that an image is capable of being obtained with a high resolution.
Specifically, when X-rays are irradiated to a charge forming layer located in the X-ray imaging panel, electric charges corresponding to X-ray energy are formed in the charge forming layer. The thus formed electric charges are read out as an electric signal. The photo-conductor layer described above acts as the charge forming layer.
A material represented by a chemical formula Bi12MO20, in which M represents at least one kind of element selected from the group consisting of Ge, Si, and Ti, has photo-conductivity and dielectric characteristics. Therefore, it has heretofore been studied to utilize the Bi12MO20 material for constituting the photo-conductor layer described above. By way of example, the use of a Bi12GeO20 material or a Bi12SiO20 material for constituting the photo-conductor layer is described in, for example, Japanese Unexamined Patent Publication No. 11(1999)-211832. Ordinarily, it has been considered as being optimum that the crystal composition of Bi12MO20be of the stoichiometric ratio. By way of example, Bi12GeO20 and Bi12SiO20 (i.e., BixGeOy and BixSiOy, in which x=12, and y=20) are described in, for example, Japanese Unexamined Patent Publication Nos. 11(1999)-237478 and 2000-249769.
Also, in, for example, “Crystal Growth and Optical Properties of Large Single-Crystals of Bismuth Silicon Oxide”, by K. Tada et al., Transactions of The Chemical Society of Japan, No. 10, pp. 1630-1639, 1981, it is described that experiments for growing a BixMOy single crystal were made with a melt composition being altered within the range of 10≦x≦14, and that a maximum photo-conduction current was obtained with a single crystal having been grown with a melt of x=11.5 (a crystal composition ratio of the single crystal, as read from a graph: approximately 12.15 and approximately 12.69). Further, in, for example, “Determination of the Diffusion Length of Charge Carriers in Nonstoichiometric Sillenite-Type Crystals by the Technique of Nonsteady-State Photocurrents”, by H. Vogt and E. Krätzig, Journal of Applied Physics, Vol. 94, No. 4, pp. 2507-2509, 2003, it is described that experiments for growing a BixGeOy single crystal were made with a GeO2 content with respect to a melt composition being altered within the range of 9.0 mol % to 20.0 mol % (expressed in terms of x, 9≦x≦20), that experiments for growing a BixSiOy single crystal were made with an Sio2 content with respect to a melt composition being altered within the range of 9.0 mol % to 18.0 mol % (expressed in terms of x, 8≦x≦20), and that alterations of μτ (i.e., the product of a carrier mobility and a life) of the thus grown single crystals were investigated.
The inventors conducted extensive research and found that, in contrast with the descriptions made in, for example, Japanese Unexamined Patent Publication Nos. 11(1999)-237478 and 2000-249769, the quantity of generated electric charges becomes large in cases where the proportion of bismuth is higher than the bismuth proportion in the crystal composition having the stoichiometric ratio, which crystal composition is ordinarily considered as being optimum. The present invention is based upon the findings described above. The materials, which are described in, for example, “Crystal Growth and Optical Properties of-Large Single-Crystals of Bismuth Silicon Oxide”, by K. Tada et al., Transactions of The Chemical Society of Japan, No. 10, pp. 1630-1639, 1981, and“Determination of the Diffusion Length of Charge Carriers in Nonstoichiometric Sillenite-Type Crystals by the Technique of Nonsteady-State Photocurrents”, by H. Vogt and E. Krätzig, Journal of Applied Physics, Vol. 94, No. 4, pp. 2507-2509, 2003, are the BixMOy single crystals. With respect to the BixMOy single crystals, which have little grain boundary, it is expected that a dark current is capable of being suppressed, and that an X-ray photo-current quantity is capable of being enhanced. However, the BixMOy single crystals have the drawbacks in that the production cost is not capable of being kept low, and it is not always possible from a technical view point to form a layer having a large area with a size of several tens of centimeters×several tens of centimeters. Therefore, the BixMOy single crystals are not appropriate for practical use. Further, as for the single crystals, the problems are encountered in that, since it is not easy to control the compositions of the single crystals, the single crystals having desired compositions are not capable of being obtained.